1. Field of the Invention
The present invention relates to a chemical vapor deposition method and apparatus for forming various deposited films such as metal films, semiconductor films and insulating films used in memory devices such as semiconductor devices and optical magnetic disks or in flat panel display devices. More particularly it relates to a chemical vapor deposition method and apparatus making use of a liquid starting material.
2. Related Background Art
Deposited films formed by chemical vapor deposition method (CVD method) using apparatus therefor (CVD apparatus) can be roughly grouped into metal films, semiconductor films and insulating films.
Of these, in the case of the semiconductor films, a film forming method capable of obtaining uniform films with less faults are desired. As for the insulating films, uniform films are desired as a matter of course and besides a film forming method that can achieve excellent coating properties on step portion is desired. This is because most insulating films are used to insulate wirings from each other in an integrated circuit (IC) or to protect its uneven surface.
In the case of the metal films, a film forming method that can achieve excellent uniformity and coating performance on step portion is also desired as in the case of the insulating films. This is because the metal films are mostly used in wiring materials for ICs and in such instances the coating properties on step portion at the holes thereof are required so that upper and lower wirings can be connected via openings called contact holes or through holes.
FIG. 1 diagrammatically illustrates the prior art CVD apparatus used for such CVD method.
In FIG. 1, reference numeral 403 denotes a reaction chamber formed of quartz or the like, provided therein substrate holders 410 that are disposed in plurality to support thereon a corresponding number of substrates 409 on which films are to be formed. Reference numeral 408 denotes an exhaust pipe, which is connected to a main pump 404 comprise of a mechanical booster pump and an auxiliary pump 405 comprised of a rotary pump and from which the inside of the reaction chamber 403 are evacuated.
As for a gas feeding system, it is provided with a bomb (bubbler) 402 having a bubbling mechanism that bubbles a liquid starting material, a gas pipe 406 through which carrier gas for the bubbling is fed, a valve 401, and a gas pipe 407 through which vaporized starting materials are fed into the reaction chamber 403.
Such a conventional CVD apparatus can make full use of its performance so long as common film formation is carried out, but is sometimes unsuitable for a CVD method that can excellently provide fine structure or form large area films as recently demanded. In other words, conventional apparatus have had some features that they are poor in general-purpose properties, i.e., adaptability to any CVD methods. This problem will be discussed below by giving an example.
Recently, as materials for the wiring of highly integrated semiconductor devices called VLSI or ULSI, aluminum films formed not by sputtering but by CVD have attracted notice. In particular, in CVD methods making use of an organic compound comprising an organoaluminum compound, it is nowadays reported that conditions for deposition greatly differ between an insulator and a conductor and hence selective deposition can be made in which aluminum is deposited only on the conductor or a semiconductor. This selective deposition of aluminum is very useful when fine integrated circuits are fabricated. In particular, in instances in which the ratio of depth to diameter (aspect ratio) of a hole is more than 1, the selective deposition can provide aluminum wiring that can not be realized by the sputtering which is a substitute technique. In the sputtering, disconnection occurs when the hole has a large aspect ratio. The reason therefor will be explained below with reference to FIGS. 2A to 2C. In FIGS. 2A and 2B, reference numeral 201 denotes a monocrystalline silicon substrate; 202, an insulating film such as silicon dioxide film; and 203, a wiring material such as aluminum.
FIG. 2A illustrates how the wiring is formed when the hole has a small aspect ratio, and FIG. 2B how the wiring is formed when the hole has a large aspect ratio of 1 or more.
In the sputtering, a hollow 204 or a void 205 is formed. On the other hand, in the selective deposition by CVD, the hole is completely filled with aluminum 303 as shown in FIG. 2C, and there is a very low probability of disconnection.
In FIG. 3, reference numeral 301 denotes a silicon substrate; 302, an insulating film such as silicon dioxide film; 303, a metallic material such as aluminum deposited by CVD; and 304, wiring of aluminum deposited by sputtering or CVD.
Thus, in the case when the wiring of a fine semiconductor device is fabricated using the CVD apparatus shown in FIG. 1, a carrier gas CGS such as hydrogen, whose pressure is reduced by means of a reducing valve 401, is fed to the bubbler 402. Most of material gases that enable the selective deposition of aluminum are liquid at room temperature, as exemplified by dimethylaluminum hydride (DMAH) and triisobutylaluminum (TIBA). For this reason, the bubbling, i.e. the step of generating bubbles in the bubbler 402 is carried out, so that a mixed gas comprised of the carrier gas and saturated vapor of organoaluminum compound such as DMAH is fed into the reaction chamber. The mixed gas is thermally decomposed on the heated semiconductor substrates 409, and aluminum is deposited on the substrate as a result of its surface reaction with the substrates.
Unreacted gas in the reaction chamber 403 is exhausted outside by means of the main pump 404 and auxiliary pump 405.
However, a change in apparatus environment such that an experimental CVD apparatus that has stably achieved the selective deposition is made up to a mass-production CVD apparatus has caused the problem that the selectivity having been achieved so far is lost.
This results in an increase in faults not only in the case of the metal films but also in the case of the semiconductor films, and results in a lowering of step portion coating properties in the case of the insulating films.
According to a finding made by the present inventors, the poor general-purpose properties is caused by the following reasons, as will be more detailed later, in the constitution of the conventional CVD apparatus.
First, the mixing ratio of the starting material liquid compound and other gas can only be very poorly controlled.
Second, a temperature change in the vicinity of the bubbler causes a change in the mixing ratio of the compound.
Third, residual gases in the bubbler cause a change in the mixing ratio of the compound.